Study Of Ignition And Combustion Process Of Boron Particles In Ducted Rockets

Boron has been considered a prime solid candidate as an energetic additive in the formulations of high-energy fuel-rich propellants due to its high potential energy release on both a gravimetric and volumetric basic. Boron particles in the primary generator of Boron-based propellant ducted rockets cannot achieve complete ignition and combustion for lack of oxygen and low temperature. The fuel-rich hot gas with large amounts of boron particles mixes rapidly with the induced ram-air from the inlet and completes its secondary combustion process in the secondary combustion chamber. And there always exists a relative speed between the boron particle and hot air stream due to the influence of the ejection mode of the fuel-rich hot gas and the ram-air. The combustion flow field around boron particles is no longer spherical symmetric. And the production and consumption of the oxide layer may be changed. So the ignition and combustion process in the convective flow will be different of those under relatively static conditions. Unfortunately, few detailed experimental studies and theoretical modelings of such cases are available at present. The ignition and combustion process in ducted rockets will be studied. And the mechanism of the process in the corresponding convective flow will be revealed in this paper. The study will lay the foundation for the energy’s efficient conversion of boron-based fuel-rich propellants in ducted rockets.A new one-dimensional model about the ignition process of single boron particle in relatively static atmosphere is processed. The gas species transport in the liquid oxide layer and the finite reaction dynamic are included in the model. The changing regularities of important parameters of the boron particle and the gas phase, such as the mass flow rate of oxygen gas, B2O3 gas and Stefan flow on the surface of the boron particle in the two typical cases, viz., the successful ignition case and the degenerate ignition case are studied. Mass fraction profiles and temperature profiles of the gas phase around the particle are also studied in the paper. And the reasons for them are analyzed in detail. The new model may real the reason for the change of the brightness of boron particle between the ignition process and the combustion process. Then the demerits of the present models, viz., the diffusion combustion models such as King Model and the kinetic combustion models such as Williams Model are analyzed in detail. The deep analysis will lay the foundation for the study of the boron particle ignition and combustion process in this paper.Considering the possible existence of the particle agglomerations in ducted rockets, the ignition process of boron particle agglomerations in relatively static atmosphere is studied systemically. A model is proposed taking into consideration the gas phase diffusion process in the boron particle agglomeration, and the heat transfer and the mass transfer process between the boron particle agglomeration and the surroundings. The influence of total ambient pressure, gas temperature, oxygen mole fraction, agglomeration radius, agglomeration porosity, and boron particle radius on ignition temperature as well as the influence of these factors on ignition delay time is analyzed in detail.A new model is proposed to describe the ignition process of single boron particle in forced convective flow. Physical and chemical processes such as the tangential shear stress of the gas flow and the finite reaction dynamic on the particle surface, the heat conduction in the inner boron nucleus are included in the model. The effect of some important gas field parameters such as the incoming gas velocity, the surrounding temperature and the total gas pressure on the ignition process of sinle boron particle in forced convective flow is studied. It is found that the incoming gas velocity, the surrounding temperature and the total gas pressure have positive effect on the ignition process of boron particles in forced convective flow.A new model of the combustion process of single boron particle in forced convective flow is proposed taking into consideration the gas flow around the particle, the gas diffusion and the global finite reaction dynamic on the particle surface. Influence factors such as the incoming gas velocity, the particle radius, the ambient oxygen mass fraction and the ambient pressure on the burning rate of single boron particle are studied. The results show that in forced convective flow, both the mass burning rate and the mass flux of the reacting boron increase with the increase in the incoming gas flow velocity, the particle radius, the ambient oxygen mass fraction and the ambient pressure. Based on large amounts of numerical simulation results, the mass flux of the reacting boron particle is modified to describe the combustion process of the reacting boron particle in forced convective flow.Then a kinetic model to describe the condension process of B2O3 gas is proposed based on the classical condension-nucleation theory. The combustion characteristics of the boron particle in forced convective flow is analysed takes into consideration the muti-step reaction kinetic, the gas flow around the boron particle, and the gas diffusion process. It is found that the flame shape of the boron particle in forced convective flow is axi-symmetric. The gas reactions in the space are relatively weak compared with the surface reactions. And the gas reaction products lay mainly in the rear of the boron particle, forming a wake flame. The surface element reactions which affect the burning rate of the boron particle increase with the increase in the incoming gas flow, the total ambient pressure, and the ambient oxygen mass fraction. But the ambient temperature has little effect on the burning rate of the element reaction.A series of experimental research on the ignition and combustion process of the boron particle in relatively static atmosphere is conducted. The condension phenomenon of B2O3 gas is observed. And the appearance of the compact oxide layer on the particle surface is obtained. Experimental research on the ignition and combustion process of the boron particle in forced convective flow is preliminary conducted. The flame shapes of the boron combustion in forced convective flow are recorded, and influence factors such as the initial particle temperature, the incoming gas velocity, and the ambient oxygen mass fraction on the ignition and combustion process of boron are studied. And analyses are done for the working mechanisms of those factors. Preliminary experimental results show that the combustion process of boron may be intensified in forced convective flow under appropriative conditions. Finally, the models proposed in the paper and the corresponding numerical simulation methods are preliminary validated based on the experimental results.